Check valves of various designs for many specific uses are generally well known. Moreover, in certain industries where the provision of a leak-proof closure from a vessel is essential, double-check valves have become accepted and, in fact, have become the standard in the industry for that reason.
One industry within which such double-check valves have become the standard is in the handling of liquified gases, and more particularly, in the liquid propane service industry. Liquid propane or LP gas is usually stored in vessels or tanks of various capacities depending on customer usage. In many cases, the tanks are filled from tank trucks or the like having a large storage capacity.
The present invention is particularly applicable to the storage tanks at the customer use facilities, and particularly relates to use within the filler of such a tank. Within the tank filler, the double-check valve is typically situated to ensure leak-proof closure of the tank and to facilitate easy filling of the tank by transfer of LP gas from the tank truck to the storage tank. Such double-check valves practically guarantee leak-proof closure by providing a double-sealing device which preferably also is accessible so that the check valves can be replaced or serviced if necessary. The safety provided by such double-check valves definitely justifies the additional cost of a double-check valve as compared with that of a single-check valve.
Even though the additional costs for a double-check valve are justified by safety, it is desirable to provide as effective a double-check valve as possible while reducing the production costs thereof. Additionally, it is particularly desirable to keep manufacturing costs down and to provide a double-check valve that ensures a leak-proof closure and does not adversely reduce filling weights by causing turbulence of the LP gas through the valve device.
One example of a double-check valve known to the prior art is shown and described in U.S. Pat. No. 3,283,778 patented Nov. 8, 1966 to E. W. Linna. The Linna patent discloses a double-check valve including an upper stem valve of fairly typical check valve construction and a lower check valve that pivots about an eccentric axis so as to maximize flow by improving flow characteristics. The major disadvantage of the Linna type check valve is the production costs associated with the particular design and the relative complexity involved in guiding the upper and lower check valves. Moreover, the construction requires a two-piece valve body which further disadvantageously provides additional leak paths.
In order to guide the upper and lower check valves of the Linna device, a spider is formed integrally with the valve body portion within which the upper and lower check valves are movable. This spider must be formed within the valve body portion, such as by forging or by machining, thereby increasing manufacturing costs. Moreover, precise bores must be machined within the spider itself for guiding the upper and lower check valves. Thus, many manufacturing or machining steps must be completed on the valve body portion during manufacturing. Each step, of course, adds to the manufacturing costs.